Cooking apparatus with transparent heating plates

ABSTRACT

Electrical apparatus for thermal processing of food includes a casing which has a transparent portion for viewing the food disposed within the casing. At least one heating element is mounted within the casing for heating the food, the heating element including a heat resistant and mechanically rigid transparent first substrate having a first surface which faces the food and a second opposite facing surface. At least one electrical resistance element is disposed on the second surface, the at least one electrical resistance element being connected to a source of electrical current. At least one second transparent substrate is disposed adjacent the second surface, the at least one second transparent substrate including a transparent infrared reflective coating. Thus, the food disposed within the casing is viewable from the exterior of the casing through at least the first substrate while the food is being heated by the first substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The instant invention is related to an electrical apparatus for thethermal processing of foodstuff, enabling the grilling, cooking,reheating or keeping warm of foodstuff, via a heating body implementingthe Joule effect.

2. Description of Background and Relevant Information

Heating food via electrical means, such as by micro-waves, induction, orstill by the Joule effect is becoming increasingly more common.

It is known that electrical toasters currently in use generally have acasing, open at its upper part, inside of which are arranged at leastone pair of parallel plates between which the bread to be toasted can beinserted. Generally, these plates hold an assembly of resistances thatare either exposed or covered in a quartz sheathing. The bread itselfcan be placed in a vertically mobile support such that the bread slicesare automatically ejected from the casing after a pre-determined heatinginterval, or after a pre-determined temperature has been attained. Thetoasting action consists of toasting the external surface of a slice ofbread, whereas the inner volume remains soft, but hot. In electricaltoasters, energy is produced by the Joule effect as a result of currentpassing through the resistances. The energy thus released is transmittedvia radiation over each of the surfaces of the bread according to afactor that is proportionate to the fourth power of the absolutetemperature in accordance with the laws of Stephan and Boltzman.

But for safety reasons, access to the heating resistances is naturallyrestricted and it is impossible to follow the progress of the operationwithout interrupting the toasting process, in order to stop it at justthe right time.

SUMMARY OF THE INVENTION

The main object of the instant invention is to overcome thisdisadvantage and to allow a visual monitoring of the progress of thebread inserted into the toaster, or of any other food processed in thesame way, by increasing visibility without reducing the heating surface.In addition, the reduction of the number of parts naturally decreasesmanufacturing costs and boosts reliability.

According to the invention, the electrical apparatus for the thermalprocessing of foodstuff having a support frame in which at least oneheating element is mounted, is characterized in that said heatingelement is constituted of an electrical resistance, connected to asource of electrical current, adjacent to at least one transparentsubstrate that is mechanically and heat resistant.

The mechanically and heat resistant substrate can be made from amonolithic glass plate or a laminated glass plate or a polymer plate.The electrical resistance can be affixed on a transparent substrate,inserted between two transparent substrates, or be constituted of aplanar conducting coating arranged on a plate.

The continuous electrically resistant coating is advantageously atransparent semi-conducting oxide coating, especially tin oxide dopedwith antimony or fluorine or indium oxide doped with tin, the resistantcoatings emitting, in the temperature range of the thermal processing, aradiation whose wave length is on the order of 7 microns.

Glass plates coated on one of their surfaces with a thin, transparentelectro-conducting coating having low emissivity and being infra-redreflecting are available commercially, the conducting coating generallybeing tin oxide doped with antimony or fluorine, or indium oxide dopedwith tin, and deposited in a thickness of 0.2 and 0.5 microns, forexample, 0.3 to 0.4 microns. Surface resistivity can range from a fewΩ/□ to a few hundred Ω/□, for example, 10 to 100 Ω/□. The conductingcoating behaves like an electrical resistance whose value is a functionof the length/width ratio. Due to the passage of current, this coatinggets heated and also heats the glass plate or, more generally, thesubstrate. It has been noted that when the surface resistance of theglass (non-coated with a coating) is on the order of 10¹² Ω/□, theemissivity of the surface beside this coating is 0.15 and 0.9 on theopposite side. A heating plate thus emits an asymmetrical radiation inthe proportions of 0.9 to 0.15. According to Kirchoff's law, radiationis only emitted over wave lengths for which the glass plate has anabsorption band. This band is located on the wave lengths comprisedbetween 5 and 20 microns. Infra-red reflecting ability is 85% beside thecoating and 4% on the opposite side. The glass-coating assembly remainstransparent to light.

Such glasses are manufactured for tin oxide coatings by the big glassmanufacturing companies such as SAINT GOBAIN. Substrates coated withindium oxide coatings are commonly found in the field of electronics.Deposits are generally carried out by vacuum cathode spray methods orliquid or gaseous phase pyrolysis methods. These glasses are mainlymarketed for the manufacture of glass panes that allow the penetrationof the visible spectrum of sunlight and conversely, reflect infra-redradiation at higher wavelengths. Such glasses are sometimes used tomanufacture windshields, allowing defogging by the Joule effect at a lowtemperature, or in the manufacture of photoelectric cells or displayunits.

The power supply in a thin coating is advantageously obtained via twofine conducting bands arranged, for example, along the two opposingedges of the plate, and to each of which is welded a conductor connectedto a voltage source.

In another embodiment, the resistance is constituted by a resistantmetallic wire pressed onto a transparent substrate or inserted betweentwo transparent substrates. Preferably, the wire is a thin wire thedisposition of which is selected to not substantially alter the opticaldensity of the resultant heating element, i.e., without substantiallydecreasing visibility.

The value of the electrical resistance of a heating element according tothe invention is selected in such a way that the surface temperature ofthe heating element is comprised between 150 and 300° C.

In the case of a toaster, the internal temperature must be on the orderof 250° C., which means that the glasses coated with the thin coatingsmust be designed in mass or treated in view of their resistance tothermal shocks. The fact that the surface temperature of the heatingelement remains comprised between 150 and 300° C. is clearly anadvantage with respect to current toasters and their extremely hightemperatures.

The heating body can itself be made of laminated glass, the conductingcoating being integrated between the two laminates. In some cases, thisconducting coating can be constituted of resisting wires that aresandwiched between the two glass coatings, without very substantiallydecreasing visibility.

In the case where the heating element is a continuous coating, it ispossible to protect the heating plate against the possibility ofelectrical short circuits and, to this end, a heat resistant andtransparent electrical insulation coating can be attached on theconducting coating, such as a coating of glass, silica, chromium oxide,titanium oxide.

Preferably, a second pair of plates, one surface of which is coated witha transparent coating having low emissivity and being infra-redreflecting is arranged in parallel with the first on the side oppositeof the food, across and at a distance from the heating element. It isthe aim of such plates to reduce energy losses towards the outside andto ensure an external apparatus temperature that poses no safety hazardsfor the user. The aim of said reflecting coating is to block theinfra-red radiation emitted by the heating element towards the outside,said reflecting coating re-emitting a part of the emitted radiationtowards the thin conducting coating. This allows losses towards theoutside to be limited and ensures that the temperature of the apparatusis such as to pose no danger for the user.

The transparent coating which reflects infra-red is mainly selected fromamong the materials previously mentioned, which simplifies theconstruction of the apparatus.

In the case of a toaster, the support receives at least two parallel andvertical heating elements such as previously defined, the bread to betoasted being then arranged in the compartment or compartmentsdemarcated by the heating plates. In case provision has been made foradditional infra-red reflecting substrates, these are naturally attachedonly to the end elements. In addition, the toaster supportadvantageously comprises an opening in its walls across from the heatingelements, such that the entirety of such heating elements remainsvisible. The support can take the shape of a traditional casing, or thatof a simple frame. It can be obtained with the possibility of enablingthe disassembly of the heating plates, facilitating their cleaning,especially in a dishwater.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the instant invention willemerge from the description of a preferred embodiment that follows,provided only as a non-restrictive example, with reference to theannexed drawings representing:

FIG. 1, a schematic diagram;

FIG. 2, a schematic perspective view of a toaster according to theinvention;

FIG. 3, a partial top view of the same toaster.

DETAILED DESCRIPTION OF THE INVENTION

The example that has been provided relates to a toaster, but theinvention can also find an application, in other grilling, cooking,reheating or warming devices.

FIG. 1 is a schematic diagram of the invention. The food to be grilled,in this case a slice of bread P, is arranged between two glass sheets1a, 1b, externally coated with conducting coatings 3a, 3b, for example,of tin oxide doped with antimony or fluorine. The coatings 3a, 3b areconnected electrically to a source of current which can be alternating,the phenomenon used being the Joule effect. The passage of the currentemits heat which toasts the bread P.

In FIG. 2, one can see that a variety of parallel plates are mounted ina casing 10. The casing 10 generally has the shape of a "U" such thatits two larger lateral surfaces 11 (only one of which can be seen in theFigure) are open at their upper ends which can be closed with a cover(not shown) leaving a central slit 2 open for introducing a slice ofbread P to be toasted. The slice of bread P can be seen, naturally, whenone looks from the top of the slit 2, but also when one looks throughthe surfaces 11.

Inside the casing 10 are assembled, from the center, two glass plates1a, 1b that are symmetrical with respect to the axis of the slit 2.According to the invention, the glass plates are coated, externally,with a thin conducting coating, respectively 3a and 3b. On the exteriorof this assembly of two plates 1a, 1b are mounted two other plates,respectively 4a and 4b which are glass plates whose surface can belarger than the surface of the conducting coatings 3a and 3b so as toavoid any accidental electrical contact. The plates 4a and 4b have, onthe side turned towards the conducting coatings 3a and 3b, a reflectingcoating respectively 5a and 5b reflecting the infra-red and adapted toreturn, at least partially, the radiation emitted thereby on theconducting coating.

Naturally, the symmetrical nature of the construction that has just beendescribed is a function of the fact that one generally wants tosimultaneously toast both sides of the same piece of bread; naturally,it would be possible to construct a toaster having a single heatingsurface in which the two surfaces could be toasted one after another.This face could obviously be horizontal.

One also sees, in the FIG. 2, a power cord 13 allowing the toaster to beconnected to a source of current, generally of 110 or 220 volts and alever 14 connected to a fork 15 allowing the slice of bread (P) to beraised or lowered at the end and at the beginning of the operations. Itis possible to provide on the conducting coatings 3a and 3b aprotection, that insulates the coating from the outside, for example, acoating of silica, chromium oxide, titanium oxide or of a transparentpolymer.

FIG. 3 represents, in a top view, the plate assembly on one side of thetoaster. One can see that on the glass plate 1b is deposited a thinconducting coating 3b made, for example, of tin oxide doped withantimony or fluorine, or indium oxide doped with tin. The thickness ofthe coating 3b is a few tenths of a micron. Across from the coating 3b,there is an infra-red reflecting coating 5b, this coating 5b beingturned towards the coating 3b; the distance between the coatings 3b and5b is on the order of about 5 mm. The coating 3b has, at its twovertical ends, electrodes 6 constituted by wide bands that allow toapply a voltage to each of the edges. The energy radiated by the coating3b towards the inside of the toaster is on the order of 1300 watts/m².The maximum wave length radiated is 7 microns which corresponds to theabsorption bands of most food items.

Although the preceding description is related to a toaster, it isunderstood that the heating method used can find an application in otherfields for heating foodstuff.

According to another embodiment (not represented), the heating iscontrolled by a photoelectric cell arranged on the outside of theapparatus and directed towards the outside thereof. This cell, whichacts on the power supply, detects the change in the color of the food,for example of the bread, during the cooking process. The instantinvention lends itself to the manufacture of transparent andaesthetically pleasing hot plates.

In all the applications, heat is transmitted by radiation or by contact.In some cases, at least a part of the apparatus may be opaque.

It is understood that numerous variations can be provided, especially bysubstitution of equivalent technical means, without however, leaving thescope of the invention

What is claimed is:
 1. An electrical apparatus for thermal processing offood, the apparatus comprising:a casing which comprises a transparentportion for viewing the food disposed within the casing; at least oneheating element mounted within the casing for heating the food, theheating element comprising a heat resistant and mechanically rigidtransparent first substrate having a first surface which faces the foodand a second opposite facing surface; at least one electrical resistanceelement disposed on the second surface, the at least one electricalresistance element being connected to a source of electrical current; atleast one second transparent substrate disposed adjacent the secondsurface, the at least one second transparent substrate comprising atransparent infrared reflective coating, wherein the food disposedwithin the casing is viewable from the exterior of the casing through atleast the first substrate while the food is being heated by the firstsubstrate.
 2. The electrical apparatus of claim 1, wherein the at leastone electrical resistance element comprises a semi-conducting oxidecoating, the semi-conductive oxide coating comprising a thickness in therange of approximately 0.2 and 0.5 microns and a surface resistivity inthe range of approximately 10 and 100 Ω/□.
 3. The electrical apparatusof claim 2, wherein the semi-conducting oxide coating comprises one oftin oxide doped with antimony, tin oxide doped with fluorine, and indiumoxide doped with tin,the semi-conductive oxide coating emitting aradiation having a wavelength of approximately 7 micron.
 4. Theelectrical apparatus of claim 1, wherein a resistivity of the at leastone electrical resistance element comprises a value wherein a surfacetemperature of the at least one heating is in the range of approximately100° C. and 300° C.
 5. The electrical apparatus of claim 1, wherein thefirst transparent substrate comprises at least one glass plate.
 6. Theelectrical apparatus of claim 1, wherein the first transparent substratecomprises a transparent coating for insulating the at least oneelectrical resistance element, the transparent coating comprising one ofsilica, chromium oxide, titanium oxide, and a polymer.
 7. The electricalapparatus of claim 1, wherein the casing comprises a toaster casing,thetoaster casing further comprising a food inserting opening at anuppermost surface portion, said at least one heating element comprisingtwo substantially parallel heating elements, said two parallel heatingelements being disposed on opposite sides of the food inserting opening,said at least one second transparent substrate comprising twotransparent second substrates with each disposed between the casing andthe first substrate.
 8. The electrical apparatus of 1, wherein thecasing further comprises at least one transparent side wall which issubstantially parallel to the at least one heating element.
 9. Theapparatus according of claim 8, wherein the at least one transparentside wall comprises a heat resistant material.
 10. An electricalapparatus for thermal processing of food, the apparatus comprising:acasing which comprises a transparent portion for viewing the fooddisposed within the casing; at least one heating element mounted withinthe casing for heating the food, the heating element comprising a heatresistant and mechanically rigid transparent first substrate having afirst surface which faces the food and a second opposite facing surface;at least one electrical resistance element comprising a semi-conductingoxide coating disposed on the second surface, the at least oneelectrical resistance element being connected to a source of electricalcurrent; at least one second transparent substrate disposed adjacent thesecond surface, the at least one second transparent substrate comprisinga transparent infrared reflective coating, wherein the food disposedwithin the casing is viewable from the exterior of the casing through atleast the first substrate while the food is being heated by the firstsubstrate.
 11. The electrical apparatus of claim 10, wherein thesemi-conductive oxide coating comprises a thickness in the range ofapproximately 0.2 and 0.5 microns and a surface resistivity in the rangeof approximately 10 and 100 Ω/□.
 12. The electrical apparatus of claim10, wherein a resistivity of the at least one electrical resistanceelement comprises a value wherein a surface temperature of the at leastone heating is in the range of approximately 100° C. and 300° C.
 13. Theelectrical apparatus of claim 10, wherein the first transparentsubstrate comprises at least one glass plate.
 14. The electricalapparatus of claim 10, wherein the first transparent substrate comprisesa transparent coating for insulating the at least one electricalresistance element, the transparent coating comprising one of silica,chromium oxide, titanium oxide, and a polymer.
 15. The electricalapparatus of claim 10, wherein the semi-conducting oxide coatingcomprises one of tin oxide doped with antimony, tin oxide doped withfluorine, and indium oxide doped with tin,the semi-conductive oxidecoating emitting a radiation having a wavelength of approximately 7micron.
 16. The electrical apparatus of claim 10, wherein the casingcomprises a toaster casing,the toaster casing further comprising a foodinserting opening at an uppermost surface portion, said at least oneheating element comprising two substantially parallel heating elements,said two parallel heating elements being disposed on opposite sides ofthe food inserting opening, said at least one second transparentsubstrate comprising two transparent second substrates with eachdisposed between the casing and the first substrate.
 17. The electricalapparatus of 10, wherein the casing further comprises at least onetransparent side wall which is substantially parallel to the at leastone heating element.
 18. The apparatus according of claim 17, whereinthe at least one transparent side wall comprises a heat resistantmaterial.
 19. An electrical apparatus for thermal processing of food,the apparatus comprising:a casing which comprises a transparent portionfor viewing the food disposed within the casing; at least one heatingelement mounted within the casing for heating the food, the heatingelement comprising a heat resistant and mechanically rigid transparentfirst substrate having a first surface which faces the food and a secondopposite facing surface; at least one electrical resistance elementdisposed on the second surface, the at least one electrical resistanceelement being connected to a source of electrical current; the at leastone electrical resistance element comprising a semi-conducting oxidecoating, the semi-conductive oxide coating comprising a thickness in therange of approximately 0.2 and 0.5 microns and a surface resistivity inthe range of approximately 10 and 100 Ω/□; at least one secondtransparent substrate disposed adjacent the second surface, the at leastone second transparent substrate comprising a transparent infraredreflective coating, wherein the food disposed within the casing isviewable from the exterior of the casing through at least the firstsubstrate while the food is being heated by the first substrate.